1. Field of the Invention
The present invention relates to the manufacturing of semiconductor circuits. More specifically, the present invention relates to the selective etching of single-crystal silicon-germanium (SiGe) in the presence of single-crystal silicon.
2. Discussion of the Related Art
One often has to etch a single-crystal silicon-germanium layer in the presence of single-crystal silicon.
For example, it has been provided to use silicon-germanium as a sacrificial layer interposed between a single-crystal semiconductor substrate and a single-crystal silicon layer to obtain a floating single-crystal silicon layer of very low thickness, for example, lower than 20 nm. This floating silicon layer is currently designated as an SON layer (for Silicon On Nothing).
Such an SON substrate can be used on forming of various components such as, for example, MOS transistors and microresonators.
For a better understanding of a problem that the present invention aims at solving, FIGS. 1A to 1C illustrate, in cross-section view, different steps of the forming of a MOS transistor on an SON substrate according to a known method.
As illustrated in FIG. 1A, a sacrificial silicon-germanium layer 5 is formed on an active region 1 of a single-crystal silicon substrate delimited by insulation areas 3. The silicon-germanium layer usually comprises from 20 to 40% of germanium. A single-crystal silicon layer 7 is then formed on sacrificial layer 5. An insulated gate 9 of a MOS transistor is formed on layer 7. A drain/source implantation, possibly preceded by a silicon expitaxy, is then performed on layer 7 on either side of gate 9.
Silicon-germanium layer 5 is then selectively removed to form an empty interval 11 between silicon region 1 and silicon layer 7. With current methods, this requires use of a protection mask 13 shown in FIG. 1B, as will be explained hereafter.
After removal of mask 13, the method carries on as illustrated in FIG. 1C by filling interval 11, for example, with an insulator 15 (but also possibly with an insulated conductive layer if a dual-gate transistor or a gate all-around transistor are desired to be formed).
A disadvantage of such a method especially lies in known methods for selectively etching sacrificial layer 5.
It has been provided to remove the silicon-germanium by means of a liquid etch with a liquid containing potassium or sodium. Such a method is very selective. However, potassium and sodium are light elements, very contaminating, which pass into the silicon of layer 7 and of region 1 as well as into insulation regions 3 and modify their conductivities. Further, the use of liquid solutions, even in the presence of surfactants, is not desirable to etch structures of very small dimensions, that is, on the order of a few nanometers.
It should be noted that the selectivity of etching of a first material in the presence of a different material can not be deduced directly from a separate study of the kinetics of the etching of each of these materials considered alone.
As an example of this fact, it has also been provided to remove the silicon-germanium by means of a dry etch using a carbon tetrafluoride plasma (CF4) in the presence of argon. Such an etching, which is not contaminating, is generally considered as strongly selective with respect to silicon, and so it is for the etching of a silicon-germanium surface close to a silicon surface. However, it can be observed that the etch selectivity becomes very insufficient in the case considered herein where a silicon-germanium layer is desired to be removed under a silicon layer.
Such a non-selectivity in a tetrafluoride plasma in the presence of argon is mainly attributed to the following reasons. First, the kinetics are astonishly modified when two materials are both present with respect to the case they are alone. Second, higher the density of the circuits processed, more important such a modification. Third, usually the kinetics of an etching is studied along a given crystalline direction and more generally along the horizontal direction, that is the [001] crystalline direction. It has been evidenced that the kinetics of a single given material along different directions are different. Additionally, the difference of selectivity between two materials varies from a direction to one another. In the present case, the selectivity is highly unpredictable as one seeks to etch away not only a first material (silicon-germanium) in the presence of a second one (silicon) but seeks additionally to etch away this first material laterally along the [110] direction while the second material is mainly exposed to the etchant horizontally along the [001] direction. These difficulties are increased when the first material lies under the second.
This is why a mask 13 for protecting the upper surface of silicon layer 7 needs to be provided during the etching of the sacrificial silicon-germanium layer. To arrange mask 13, a sequence of photolithography steps needs to be carried out. This sequence uses a mask which is not self-aligned with respect to gate 9, with the well-known disadvantages of the insertion of a non-self-aligned masking step.
However, even using the mask 13, one observes a lower selectivity than the expected one, due to the unpredictable effects of the combined presence of silicon and silicon-germanium with a high density and of the exposition of the silicon layer to the etchant laterally, along direction [110].
As an another example of the above facts, the article “Chemical vapor etching of Si, SiGe and Ge with HCl; application to the formation of thin relaxation of threading dislocations” by Y. Bogumilowicz and al. published in Semiconductor Science and Technology 20(2005)127-134 studies the etching kinetics of silicon and of silicon-germanium. The kinetics are studied separately for each material. They are studied only along the single [001] direction. According to the numerous above-exposed well-known phenomenon appearing when a first given material has to be etched away in the presence of and under a second and different material and to be etched away along the unusual lateral direction while the second material is exposed horizontally and laterally to the etchant, this article gives no teachings about the kinetics when both silicon and silicon-germanium are present. In addition, the way in which the kinetics would vary while the density of the elements exposed to the etchant is high is unknown.